Subscriber line testing is a required function of all modern telephone systems. The testing is necessary to monitor the status of the line and to detect problems requiring repair. Line impedance between the conductor pair and between each conductor and ground represents a basic parameter enabling a conclusion to be drawn about the status of the line, i.e, whether it is shorted, or whether there is leakage impedance, etc.
The prior art of line testing includes two major approaches for determining the parameters of line impedance (capacitance, inductance, resistance):
1) DC charging of the line
2) AC signal injection (or combination of these two approaches).
The first of these approaches involves charging the line for an interval, and then discharging it via a known resistance. The discharge and its characteristic curve are recorded to obtain the RC constant, from which the capacitance can be derived. The difficulty with this approach is that the line may have distributed capacitance, complicating the mathematical derivation of the true value. This is typically the problem in telephone lines.
In addition, the time required for performance of this test is lengthy, because there may be a long interval for a full line discharge. An example of this approach is given in U.S. Pat. No. 4,054,760 to Morgen, although the testing method described therein does not involve parameter derivation, but uses impedance meter deflection to indicate the status of the line.
The second approach outlined above uses AC signal injection and permits impedance derivation by measurement of two parameters: 1) Phase shift; and 2) Absolute value of impedance.
The impedance value is derived from data gathered at exact frequencies, and thus a pure sinusoid signal is required. Usually a signal generator is used to generate the sinusoidal signal. An example of this approach is given in U.S. Pat. No. 4,620,069 to Godwin et al., using sinusoidal waveform analysis.
Use of this type of signal generator has drawbacks at the time the measurement is made on communication lines, due to the variable range of the impedances, and the requirement that the generator drive the signal into the line where a low impedance may exist. In subscriber line testing, this method requires a variable frequency signal generator with high power output to force the sine wave over the length of the loop impedance. This method requires special, expensive equipment.
In U.S. Pat. No. 4,581,493 to Gazzo et al, there is disclosed a computer-control led system for accessing subscriber lines and performing various tests and reporting line conditions. The tests include measurement of line resistance and capacitance.
In U.S. Pat. No. 4,864,598 to Lynch et al, there is disclosed a loop status verification system comprising circuitry for transmitting a test signal at a sub-audible frequency over the line, and monitoring changes in the test signal, to verify line integrity. A loop current detection circuit detects the level of loop current, which is related to the loop impedance, and this information is used to generate a tone signal with a specific sub-audible frequency which is transparent to normal loop operation. The generated tone can be used to verify line integrity at a telephone central office.
As stated previously, the measurement of subscriber line impedance provides important information in determining the line status, which is the goal of all line testing methods.
Therefore, it would be desirable to provide a simple, quick, reliable and easy-to-operate device and method for subscriber line impedance testing, without using costly equipment.